Brood size and immunity costs in zebra finchesTaeniopygia guttata

Abstract

Birds rearing experimentally enlarged broods have lower antibody responses to a novel antigen, and we tested three hypotheses that could explain this result. We used zebra finchesTaeniopygia guttatainoculated with sheep red blood cells (SRBC) as a study system, for which this trade‐off was previously demonstrated. 1. Compensatory cellular immunity: The humoral immune response is slow, and removal of SRBC through up‐regulated cellular immunity could pre‐empt an antibody response. However, cellular immune response to PHA decreased with increasing brood size, allowing rejection of this hypothesis. 2. Costs of antibody‐production: Chicks in large broods grow less well, and birds with large broods may allocate resources to chicks instead of antibodies when these are costly. Compared to saline controls, SRBC suppressed metabolic rate in the hours following immunisation, but there was no effect in the following night, or at any time 4 and 8 days later. Fitness costs were measured by repeatedly immunising parents with SRBC while rearing young. Chick growth, parental condition, and subsequent reproduction of the parents were not affected by SRBC. We conclude that the costs of antibody formation cannot explain the trade‐off between brood size and antibody responsiveness. 3. Costs of immune system maintenance: Maintaining a system enabling antibody‐formation may be very costly, and birds rearing large broods may have down‐regulated this system. Based on this hypothesis we predicted that antibody formation would still be reduced in parents rearing large broods when immunised after rearing the chicks. Our results confirmed this prediction, and we suggest that birds rearing large broods have lower antibody responses because they economised on the maintenance costs of the immune system.